Which one of the following, if true, would most seriously undermine the explanation proposed by the author in the third paragraph? A. A number of songbird species related to the canary have a shorter life span than the canary and do not experience neurogenesis. B. The brain size of several types of airborne birds with life spans similar to those of canaries has been shown to vary according to a two-year cycle of neurogenesis. C. Several species of airborne birds similar to canaries in size are known to have brains that are substantially heavier than the canary's brain. D. Individual canaries that have larger-than-average repertoires of songs tend to have better developed muscles for flying. E. Individual canaries with smaller and lighter brains than the average tend to retain a smaller-than-average repertoire of songs.